Unraveling Corrosion Inhibition Through Integrated Electrochemical, Quantum Chemical and Molecular Simulation Approaches for Mild Steel in 1 M HCl by a Pyrazole-Based Carboxamide Inhibitor

This study provides a detailed assessment of the corrosion-inhibiting performance of a previously synthesized pyrazole derivative (R9) for mild steel, using both experimental and theoretical methods. Electrochemical measurements, including potentiodynamic polarization and electrochemical impedance spectroscopy, showed that R9 achieved a maximum inhibition efficiency of 81% at a concentration of 10−3 M in 1 M hydrochloric acid. This improvement was reflected in the marked decrease in corrosion current density from 604 to 94 µA·cm−2. The inhibitor displayed mixed-type behavior, influencing both anodic and cathodic corrosion reactions. This was confirmed by the small shift in corrosion potential recorded with and without R9, along with the increase in polarization resistance and the enhanced protection of the steel surface. Inductively coupled plasma spectrometry was used to measure dissolved metal ions, while scanning electron microscopy combined with energy-dispersive X-ray spectroscopy confirmed the formation of an adsorbed protective film on the steel surface. These findings further supported the effectiveness of R9 and agreed well with the electrochemical results. In the theoretical part, quantum chemical calculations on the isolated inhibitor R9 and the Fe-R9 complex (density functional theory, molecular electrostatic potential, Fukui indices, and atomic charges) were coupled with molecular simulations based on both molecular dynamics and Monte Carlo methods to provide a comprehensive understanding of the corrosion inhibition mechanism. The findings from the electronic structure studies, active site predictions, and adsorption analyses demonstrated effective and stable complexation of the R9 molecule with the steel. The results revealed an excellent correlation between the experimental and theoretical methods employed, highlighting the significance and robustness of the present study.